Method of removing carbon dioxide from a fluid stream and fluid separation assembly

ABSTRACT

The invention relates to a method of removing carbon dioxide from a fluid stream by a fluid separation assembly. The fluid separation assembly has a cyclonic fluid separator with a tubular throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device. The separation vessel has a tubular section positioned on and in connection with a collecting tank. In the method, a fluid stream with carbon dioxide is provided. Subsequently, a swirling motion is imparted to the fluid stream so as to induce outward movement. The swirling fluid stream is then expanded such that components of carbon dioxide in a meta-stable state within the fluid stream are formed. Subsequently, the outward fluid stream with the components of carbon dioxide is extracted from the cyclonic fluid separator and provided as a mixture to the separation vessel. The mixture is then guided through the tubular section towards the collecting tank while providing processing conditions such that solid carbon dioxide is formed. Finally, solidified carbon dioxide is extracted.

FIELD OF THE INVENTION

The invention relates to a method of removing carbon dioxide from afluid stream. In particular, embodiments of the present invention relateto a method of removing carbon dioxide from a natural gas stream. Theinvention further relates to a fluid separation assembly.

BACKGROUND OF THE INVENTION

Natural gas from storage or production reservoirs typically containscarbon dioxide (CO₂). Such a natural gas is denoted as a “sour” gas.Another species denoted as “sour” in a fluid stream is hydrogen sulphide(H₂S). A fluid stream without any of aforementioned sour species isdenoted as a “sweet” fluid.

CO₂ promotes corrosion within pipelines. Furthermore, in somejurisdictions, legal and commercial requirements with respect to amaximum concentration of CO₂ in a fluid stream may be in force.Therefore, it is desirable to remove CO₂ from a sour fluid stream.

Fluid sweetening processes, i.e. a process to remove a sour species likecarbon dioxide from a fluid stream, are known in the art. Such processestypically include at least one of chemical absorption, physicalabsorption, adsorption, low temperature distillation, also referred toas cryogenic separation, and membrane separation.

The use of such methods for removing carbon dioxide from a fluid streamis complex and expensive.

SUMMARY OF THE INVENTION

It is desirable to have a method of removing carbon dioxide from a fluidstream which operates more efficiently than the methods mentioned above.For this purpose, an embodiment of the invention provides a method ofremoving carbon dioxide from a fluid stream by a fluid separationassembly comprising:

-   -   a cyclonic fluid separator comprising a throat portion arranged        between a converging fluid inlet section and a diverging fluid        outlet section and a swirl creating device configured to create        a swirling motion of the carbon dioxide containing fluid within        at least part of the cyclonic fluid separator, the converging        fluid inlet section comprising a first inlet for fluid        components and the diverging fluid outlet section comprising a        first outlet for carbon dioxide depleted fluid and a second        outlet for carbon dioxide enriched fluid;

-   a separation vessel having a first section in connection with a    collecting tank, the first section being provided with a second    inlet connected to the second outlet of the cyclonic fluid    separator, and the collecting tank being provided with a third    outlet for solidified carbon dioxide, wherein said separation vessel    is operated at a pressure and temperature combination that is at or    in the vicinity of the phase boundary between a vapour/liquid/solid    coexistence region (IVb) and the vapour/solid coexistence region    (IVa);    the method comprising:    -   providing a fluid stream at the first inlet, the fluid stream        comprising carbon dioxide;    -   imparting a swirling motion to the fluid stream so as to induce        outward movement of at least one of condensed components and        solidified components within the fluid stream downstream the        swirl creating device and to form an outward fluid stream;    -   expanding the swirling fluid stream, so as to form components of        liquefied carbon dioxide in a meta-stable state within the fluid        stream, and induce outward movement of the components of        liquefied carbon dioxide in the metastable state under the        influence of the swirling motion;    -   extracting the outward fluid stream comprising the components of        liquefied carbon dioxide in the meta-stable state from said        cyclonic fluid separator through the second outlet;    -   providing the extracted outward fluid stream as a mixture to the        separation vessel through the second inlet;    -   guiding the mixture through the first section of the separation        vessel towards the collecting tank, while providing processing        conditions in the first section such that solidified carbon        dioxide is formed out of the components of liquefied carbon        dioxide in the meta-stable state;    -   extracting the solidified carbon dioxide through the third        outlet.

In an embodiment, the invention further relates to a fluid separationassembly for removing carbon dioxide from a fluid stream, the fluidseparation assembly comprising:

-   -   a cyclonic fluid separator comprising a throat portion arranged        between a converging fluid inlet section and a diverging fluid        outlet section and a swirl creating device configured to create        a swirling motion of the carbon dioxide containing fluid within        at least part of the separator, the converging fluid inlet        section comprising a first inlet for fluid components and the        diverging fluid outlet section comprising a first outlet for        carbon dioxide depleted fluid and a second outlet for carbon        dioxide enriched fluid;    -   a separation vessel having a first section in connection with a        collecting tank, the section being provided with a second inlet        connected to the second outlet of the cyclonic fluid separator,        and the collecting tank being provided with a third outlet for        solidified carbon dioxide, wherein said separation vessel is        operated at a pressure and temperature combination that is at or        in the vicinity of the phase boundary between a        vapour/liquid/solid coexistence region (IVb) and the        vapour/solid coexistence region (IVa);        wherein the fluid separation assembly is arranged to:    -   receive a fluid stream comprising carbon dioxide at the first        inlet;    -   impart a swirling motion to the fluid stream so as to induce        outward movement of at least one of condensed components and        solidified components within the fluid stream downstream the        swirl creating device and to form an outward fluid stream;    -   expand the swirling fluid stream, so as to form components of        liquefied carbon dioxide in a meta-stable state within the fluid        stream, and induce outward movement of the components of        liquefied carbon dioxide in the meta-stable state under the        influence of the swirling motion;    -   extract the outward fluid stream comprising said components of        liquefied carbon dioxide in the meta-stable state from the        cyclonic fluid separator through the second outlet;    -   provide the extracted outward fluid stream as a mixture to the        separation vessel through the second inlet;    -   guide the mixture through the first section of the separation        vessel towards the collecting tank, while providing processing        conditions in the first section such that solidified carbon        dioxide is formed out of the components of liquefied carbon        dioxide in the meta-stable state;    -   enable extraction of the solidified carbon dioxide through the        third outlet.

Throughout the description, the term “fluid” is used. This term is usedto refer to liquid and/or gas.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts and inwhich:

FIG. 1 schematically depicts a longitudinal sectional view of a cyclonicfluid separator that may be used in embodiments;

FIG. 2 schematically depicts a cross-sectional view of a separationvessel that may be used in embodiments;

FIGS. 3 a, 3 b depict an exemplary phase diagram of a natural gascontaining carbon dioxide in which schematically different embodimentsof the method are visualised,

FIGS. 4, 5, 6, 7, 8 a and 8 b schematically depict further embodiments.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a longitudinal sectional view of a cyclonicfluid separator 1 that may be used in embodiments of the invention. Sucha cyclonic fluid separator is described in more detail in internationalpatent application WO03/029739. It must be understood that, inembodiments of the invention, also cyclonic fluid separators of adifferent type may be used, e.g. a cyclonic fluid separator as describedin WO99/01194, WO2006/070019 and WO00/23757.

The cyclonic fluid separator 1 comprises a converging fluid inletsection 3, a diverging fluid outlet section 5 and a tubular throatportion 4 arranged in between the converging fluid inlet section 3 anddiverging fluid outlet section 5. The cyclonic fluid separator 1 furthercomprises a swirl creating device, e.g. a number of swirl impartingvanes 2, configured to create a swirling motion of the fluid within atleast part of the cyclonic fluid separator 1.

The cyclonic fluid separator 1 comprises a pear-shaped central body 11on which the swirl imparting vanes 2 are mounted and which is arrangedcoaxial to a central axis I of the cyclonic separator 1 and inside thecyclonic separator such that an annular flow path is created between thecentral body 1 and separator housing 20.

The width of the annulus is designed such that the cross-sectional areaof the annulus gradually decreases downstream of the swirl impartingvanes 2 such that in use the fluid velocity in the annulus graduallyincreases and reaches a supersonic speed at a location downstream of theswirl imparting vanes 2.

The cyclonic separator 1 further comprises a tubular throat portion 4from which, in use, the swirling fluid stream is discharged into adiverging fluid separation chamber 5 which is equipped with a centralprimary outlet conduit 6 for gaseous components and with an outersecondary outlet conduit 7 for condensables enriched fluid components.The central body 1 has a substantially cylindrical elongated tailsection 8 on which an assembly of flow straightening blades 19 ismounted. The central body 11 has a largest outer width or diameter2R_(o max) which is larger than the smallest inner width or diameter2R_(n min) of the tubular throat portion 4.

The tubular throat portion 4 comprises the part of the annulus 3 havingthe smallest cross-sectional area. The maximum diameter of the centralbody 1 is larger than the minimum diameter of the tubular throat portion4.

The converging fluid inlet section 3 comprises a first inlet 10. Thediverging fluid outlet section 5 comprises a first outlet 6 and a secondoutlet 7.

The function of the various components of the cyclonic fluid separator 1will now be explained with respect to a case in which the cyclonic fluidseparator 1 is used to separate carbon dioxide from a fluid streamcomprising carbon dioxide in accordance with an embodiment of theinvention.

The fluid stream comprising carbon dioxide is fed through the firstinlet 10 in the converging fluid inlet section 3. In an embodiment ofthe invention, the fluid stream comprises a mole percentage carbondioxide larger than 10%. The swirl imparting vanes 2 create acirculation in the fluid stream and are oriented at an angle α relativeto the central axis of the cyclonic fluid separator 1, i.e. the axisaround which the cyclonic fluid separator 1 is about rotationallysymmetric. The swirling fluid stream is then expanded to highvelocities. In embodiments of the invention, the number of swirlimparting vanes 2 is positioned in the throat portion 4. In otherembodiments, of the invention, the number of swirl imparting vanes 2 ispositioned in the converging fluid inlet section 3. Again, the centralbody 11 has a largest outer width or diameter 2R_(o max) which is largerthan the smallest inner width or diameter 2R_(n min) of the tubularthroat portion 4.

In embodiments of the invention, the swirling fluid stream has atransonic velocity. In other embodiments of the invention, the swirlingfluid stream may reach a supersonic velocity. The expansion is performedrapidly. With respect to an expansion two time scales may be defined.

The first time scale is related to a mass transfer time t_(eq), i.e. atime associated with return to equilibrium conditions. The t_(eq)depends on the interfacial area density in a two-phase system, thediffusion coefficient between the two phases and the magnitude of thedeparture from equilibrium. The t_(eq) for a liquid-to-solid transitionis typically two orders of magnitude larger than for a vapour-to-liquidtransition.

The second time scale is related to an expansion residence time t_(res)of the fluid in the device. The t_(res) relates to the average speed ofthe fluid in the device and the axial length of the device along whichthe fluid travels. An expansion is denoted as ‘rapid’ when

$\frac{t_{eq}}{t_{res}} > 1.$

Due to the rapid expansion which causes a high velocity of the fluidstream, the swirling fluid stream may reach a temperature below 200 Kand a pressure below 50% of a pressure at the first inlet 10 of theconverging inlet section 3. As a result of aforementioned expansion,carbon dioxide components are formed in a meta-stable state within thefluid stream. In case the fluid stream at the inlet section 3 is a gasstream, the carbon dioxide components will be formed as liquefied carbondioxide components. In case the fluid stream at the inlet section 3 is aliquid stream, hydrocarbon vapours will be formed whilst the majority ofcarbon dioxide components remain in liquid form. In the tubular throatportion 4, the fluid stream may be induced to further expand to highervelocity or be kept at a substantially constant speed.

In the first case, i.e. expansion of the fluid stream to highervelocity, aforementioned formation of carbon dioxide components isongoing and particles will gain mass. Preferably the expansion isextended to a solid coexistence region (region IVa or IVb in FIGS. 3 a,3 b). However solidification will be delayed with respect toequilibrium, since the phase transition from liquid to solid isassociated with a barrier of the free energy of formation. As will befurther discussed with respect to FIGS. 3 a, 3 b, a portion of thecarbon dioxide may solidify.

In case the fluid stream is kept at substantially constant speed, carbondioxide component formation is about to stop after a defined relaxationtime. In both cases, i.e. expansion of the fluid stream to highervelocity and keeping the fluid stream at a substantially constant speed,the centrifugal action causes the carbon dioxide particles to drift tothe outer circumference of the flow area adjacent to the inner wall ofthe housing of the cyclonic fluid separator 1 so as to form an outwardfluid stream. In this case the outward fluid stream is a stream of acarbon dioxide enriched fluid, the carbon dioxide components thereinbeing liquefied and/or partly solidified.

Downstream of the tubular throat portion 4, the outward fluid streamcomprising the components of carbon dioxide in aforementionedmeta-stable state is extracted from the cyclonic fluid separator 1through the second outlet 7 of the cyclonic fluid separator 1. Othercomponents within the fluid stream not being part of aforementionedoutward fluid stream are extracted from the cyclonic fluid separator 1through first outlet 6 of the cyclonic fluid separator 1.

FIG. 2 schematically depicts a cross-sectional view of a separationvessel 21 that may be used in embodiments of the invention. Theseparation vessel 21 has a first section, further referred to as tubularsection 22, with, in use, a substantially vertical orientationpositioned on and in connection with a collecting tank 23. Thecollecting tank 23 is provided with a third outlet 28 and a fourthoutlet 26. The tubular section 22 is provided with a second inlet 25 anda fifth outlet 29. The second inlet 25 is connected to the second outlet7 of the cyclonic fluid separator 1. In an embodiment, the second inlet25 is arranged to provide a tangential fluid stream into the separationvessel 21, e.g. the second inlet 25 is arranged tangent to thecircumference of the separation vessel 21. The separation vessel 21further comprises a cooling arrangement, in FIG. 2 schematicallyrepresented by reference number 31, and a separation arrangement, inFIG. 2 schematically represented by reference number 33.

The function of the various components of the separation vessel 21 willnow be explained with respect to a case in which the separation vessel21 is used in a method of removing carbon dioxide from a fluid stream inaccordance with an embodiment of the invention.

The cooling arrangement 31 is configured to provide a predeterminedtemperature condition in the separation vessel 21. The temperaturecondition is such that it enables solidification of the carbon dioxideenriched fluid, which enters the separation vessel 21 through the secondinlet 25 as a mixture. In other words, the temperature within theseparation vessel 21 should remain below the solidification temperatureof carbon dioxide, the latter being dependent on the pressure conditionsin the separation vessel 21.

Within the separation vessel 21, a mixture comprising carbon dioxideoriginating from the second outlet 7 of the cyclonic fluid separator 1is split in at least three fractions. These fractions are a firstfraction of gaseous components, a second fraction of hydrocarbon,predominantly in a liquid state, and a third fraction of carbon dioxide,predominantly in a solid state.

The first fraction is formed by gaseous components which are draggedalong with the liquids exiting the second outlet 7. The coolingarrangement 31 is configured to keep the temperature within theseparation vessel 21 below the solidification temperature of the fluid.The gaseous components do not contain much carbon dioxide as most carbondioxide will be dissolved in the mixture liquid, as will be explained inmore detail with reference to FIG. 3. The carbon dioxide depletedgaseous components may leave the separation vessel 21 through the fifthoutlet 29.

The vessel 21 may be equipped with one or more inlets 25 which arepositioned tangent to the perimeter of the vertical section 22, suchthat a rotational flow in section 22 results. Furthermore the top gasoutlet 29 may extent as a vertical pipe in said vertical section 22 asto form a so-called vortex finder. The edge of said vortex finder is ata vertical lower position compared to the vertical position of theinlet(s) 25. This is explained in more detail below with reference toFIG. 7.

The edge of the vortex finder (i.e. lowest part of the gas outlet 29),is below the inlet 25 to allow the components that enter through theinlet 25 to separate before reaching the edge of the vortex finder. Sothis distance is provided to prevent liquids and solids from enteringthe vortex finder. The liquids and solids will be forced to the outerperimeter due to the rotational forces and will not enter the gas outlet29.

The sections 22 and 23 of vessel 21 may be physically separated by aconical shaped vortex breaker of which the outer perimeter has aclearance C with respect to the inner perimeter of the vertical section22. This clearance C can range typically from 0.05 to 0.3 times theinner diameter of section 22. This is explained in more detail belowwith reference to FIG. 7.

As a result of solidification of carbon dioxide out of the liquid withinthe mixture, a phenomenon which will be explained in more detail withrespect to FIG. 3, the mixture, which no longer holds gaseouscomponents, may be split in a liquid component containing hydrocarbonand a solid component of carbon dioxide by means of a separationarrangement 33. Possible separation arrangements 33 include a gravityseparator, a centrifuge and a hydro cyclone. In case a gravity separatoris used, it preferably comprises a number of stacked plates. In case acentrifuge is used, it preferably comprises a stacked disc bowl. Theseparation arrangement 33 in the separation vessel 21 is configured toenable carbon dioxide enriched hydrocarbon liquid components to leavethe separation vessel 21 through the fourth outlet 26, and to enablesolidified carbon dioxide to leave the separation vessel 21 through thethird outlet 28.

In an embodiment, the fluid separation assembly further comprises ascrew conveyor or scroll type discharger 35 in connection with the thirdoutlet 28. The scroll type discharger 35 is configured to extract thesolidified carbon dioxide from the separation vessel 21.

In yet another embodiment, interior surfaces of elements of the fluidseparation assembly being exposed to the fluid, i.e. cyclonic fluidseparator 1, separation vessel 21 and the one or more tubes or the likeconnecting the second outlet 7 of the cyclonic fluid separator 1 and thesecond inlet 25 of the separation vessel 21, are provided with anon-adhesive coating. The non-adhesive coating prevents adhesion ofsolidified fluid components, i.e. carbon dioxide, on aforementionedinterior surfaces. Such adhesion would decrease the efficiency of thefluid separation assembly.

FIGS. 3 a, 3 b show an exemplary phase diagram of a natural gascontaining carbon dioxide in which schematically different embodimentsof the method according to the invention are visualised. The phases arerepresented as a function of pressure in bar and temperature in degreesCelsius. In this particular case, the natural gas contains 71 mol % CO₂.Additionally, the natural gas contains 0.5 mol % nitrogen (N₂), 0.5 mol% hydrogen sulphide (H₂S), 27 mol % C1, i.e. hydrocarbons with a singlecarbon atom therein, and 1 mol % C2, i.e. hydrocarbons with two carbonatoms therein. The phases are labelled as follows: V=vapour, L=liquid,C=solid CO₂. Areas of different coexisting phases are separated bycalculated phase boundaries.

In FIG. 3 a, the condition of the fluid stream at the first inlet 10 ofthe cyclonic fluid separator 1 schematically depicted in FIG. 1corresponds to the coordinate of 80 bar and −40° C., denoted by [START]in the diagram of FIG. 3 a. The isentropic trajectory along arrow A isin the liquid region (II), whereas the isentropic trajectory along arrowB is in the vapour/liquid coexistence region (III). As a result of theexpansion in the coexistence region (III), a meta-stable state in theliquid/vapour regime may be reached while following arrow B, until phasetransition occurs at a certain super saturated condition. The resultingevaporation process will then restore equilibrium conditions. Furtherexpansion of the fluid stream along the arrow C may result in the fluidto reach a meta-stable state in the vapour/liquid/solid coexistenceregion (IVb) or in the vapour/solid coexistence region (IVa). Eventhough along the expansion trajectory denoted with arrow C, a phasetransition to form solid carbon dioxide will not occur instantaneously,the carbon dioxide fraction in the vapour will deplete, while morecarbon dioxide dissolves in the liquid. In embodiments of the invention,the fluid stream may be separated by a cyclonic fluid separator, e.g. acyclonic fluid separator as described in International patentapplication WO2006/070019, in a carbon dioxide enriched fluid stream anda carbon dioxide depleted fluid stream at the end of the expansiontrajectory denoted by arrow C. The separated, carbon dioxide enrichedfluid is in a state of non-equilibrium, which will only last for alimited period of time, in the order of 10 milliseconds. Therefore thecarbon dioxide enriched fluid is recompressed in the second outlet 7 ofthe diverging outlet section 5 of the cyclonic fluid separator 1 anddischarged via the second outlet 7 to the separation vessel 21,preferably within said time period that the meta-stable state exists. Abreakdown of said meta-stable state results in solid formation which inpractice means that dissolved carbon dioxide in the liquid solidifies.As a result of the solidification of carbon dioxide, latent heat isreleased causing the temperature of the fluid to rise. Therefore theseparated, carbon dioxide enriched fluid entering the separation vessel21, may be cooled in order to ensure that the fluid remains in thevapour/solid or vapour/liquid/solid coexistence region. Said process ofcooling and recompressing the carbon dioxide enriched fluid is denotedby arrow D. In embodiments of the invention, the process of furthersolidification takes place in the separation vessel 21. The state of thefluid at a newly developed equilibrium within the separation vessel 21is denoted as [END]. Solidified carbon dioxide is removed through thethird outlet 28 as described above.

In FIG. 3 b, the condition of the fluid stream at the first inlet 10 ofthe cyclonic fluid separator 1 schematically depicted in FIG. 1corresponds to the coordinate of about 85 bar and about 18° C., denotedby [START] in the diagram of FIG. 3 b. The isentropic trajectory alongarrow A′ is in the vapour region (I), whereas the isentropic trajectoryalong arrow B′ is in the vapour/liquid coexistence region (III). As aresult of the expansion in the coexistence region (III), a meta-stablestate in the liquid/vapour regime may be reached while following arrowB′, until phase transition occurs at a certain super-cooled condition.The resulting condensation process will then restore equilibriumconditions. Further expansion of the fluid stream along the arrow C′ mayresult in the fluid to reach a meta-stable state in thevapour/liquid/solid coexistence region (IVb) or in the vapour/solidcoexistence region (IVa). Even though along the expansion trajectorydenoted with arrow C′, a phase transition to form solid carbon dioxidewill not occur instantaneously. In embodiments of the invention, thefluid stream is separated by the cyclonic fluid separator 1 in a carbondioxide enriched fluid stream and a carbon dioxide depleted fluid streamat the end of the expansion trajectory denoted by arrow C′, a processdescribed above with reference to FIG. 1. Additionally, further detailswith respect to such a process may be found in international applicationWO03/029739. The separated, carbon dioxide enriched fluid is in a stateof non-equilibrium, which will only last for a limited period of time,in the order of 10 milliseconds. Therefore the carbon dioxide enrichedfluid is recompressed in the diverging outlet section 5 of the cyclonicfluid separator 1 and discharged via the second outlet 7 to theseparation vessel 21, preferably within said time period that themeta-stable state exists. A breakdown of said meta-stable state resultsin solid carbon dioxide formation from the liquefied part of the fluidstream. As a result of the solidification of carbon dioxide, latent heatis released causing the temperature of the fluid to rise. Therefore theseparated, carbon dioxide enriched fluid entering the separation vessel21, may be cooled in order to ensure that the fluid remains in thevapour/solid or vapour/liquid/solid coexistence region. Said process ofcooling and recompressing the carbon dioxide enriched fluid is denotedby arrow D′.

In embodiments of the invention, the process of solidification takesplace in the separation vessel 21. The state of the fluid at a newlydeveloped equilibrium within the separation vessel 21 is denoted as[END]. Again, solidified carbon dioxide is removed through the thirdoutlet 28 as described above.

For the examples provided above with reference to FIGS. 3 a and 3 b, themaximum carbon dioxide solid fraction for a given temperature T isobtained at a pressure P intersecting the phase boundary between regionsLVC (IVb) and VC (Iva).

As explained above, the function of the separation vessel 21 is toremove a maximum amount of carbon dioxide in the solid phase. Therefore,according to an embodiment, the separation vessel 21 is operated at apressure P and a temperature T at or close to the phase boundary betweenregions LVC (IVb) and VC (IVa). This phase boundary is shown in FIGS. 3a and 3 b.

In the example provided in FIGS. 3 a and 3 b, this phase boundary iscrossed by arrow D, which represents the process of cooling andrecompressing the carbon dioxide enriched fluid as it takes place in theseparation vessel 21. As shown, the state of the fluid at a newlydeveloped equilibrium within the separation vessel 21 is denoted as[END]. According to the embodiment described here, [END] is chosen at orclose to the phase boundary between regions LVC (IVb) and VC (IVa). Thisis done as the amount of solid carbon dioxide is at its maximum at thisphase boundary.

In this embodiment, the term “close to the phase boundary” is used toindicate a margin in the temperature of ±5° C. with respect to theindicated phase boundary and a margin in the pressure of ±2 or ±5 bar ora margin of 10% or 20% with respect to the indicated phase boundary.

Thus according to an embodiment, the separation vessel 21 is operated ata pressure P within 5 bar and at a temperature T within 5° C. within thephase boundary between regions LVC (IVb) and VC (IVa).

This conditions may be controlled by controlling the pressure andtemperature within the separation vessel 21. The temperature in theseparation vessel 21 may be controlled by using cooling arrangement 31.The pressure in the separation vessel 21 may be controlled by a pressureregulating valve which is located in the gas outlet stream 29.

According to an embodiment, the separation vessel 21 is operated at apressure and temperature combination that is at or in the vicinity ofthe phase boundary between the vapour/liquid/solid coexistence region(IVb) and the vapour/solid coexistence region (IVa).

According to the examples provided with reference to FIGS. 3 a and 3 b,the separation vessel 21 may be operated at a pressure in the range of5-25 bar. The proposed temperate range for these examples is in therange of −70° C. to −90° C.

FIGS. 4, 5 and 6 schematically depict a further embodiment, in which thescrew conveyor or scroll type discharger 35 is replaced with aperforated screen 40. FIG. 4 shows a side-view of such a perforatedscreen 40, where FIG. 5 shows a top view of such a perforated screenaccording to a possible embodiment. FIG. 6 schematically depicts such aperforated screen 40 in combination with separation vessel 21.

According to this embodiment the solidified carbon dioxide is removedfrom the separation vessel 21 by means of a perforated screen 40comprising tapered openings/slots or conical holes. The perforatedscreen 41 may be heated and a pressure difference may be maintainedbetween a feed side 42 and a collection side 43, such that the pressureat the feed side is always higher than or equal to the pressure at thecollection side.

The perforated screen 40 may be provided with a plurality ofperforations or openings 41. The openings 41 may be rectangularopenings, openings formed as slots, or may be circular openings as shownin FIG. 5.

The solidified carbon dioxide particles that leave the separation vessel21 through the third outlet 28 are transported to the feed side 42 ofthe perforated screen 40, as shown in FIG. 4. The solidified carbondioxide particles are transported through the openings 41 from the feedside 42 to the collection side 43 of the perforated screen 40. The sizeand shape of the openings 41 are such that, in use, the solidifiedcarbon dioxide particles fill the openings 41 and form a layer ofsolidified carbon dioxide, thereby preventing transport of gases andliquids from the collection side 43 to the feed side 42.

To create such a layer of solidified carbon dioxide and thereby avoidseepage flow of liquid or gas through the openings 41 from thecollection side 43 to the feed side 42, the openings 41 may be providedwith a tapered shape or conical shape, i.e. the openings 41 are providedwith a cross section at the feed side 41 that is larger than a crosssection of the opening 41 at the collection side 43. This is shown inFIG. 4.

An angle of convergence α of these openings 41 can be in the range of 5°and 30° with respect to a longitudinal axis 44 of the opening 41.According to a further embodiment, the angle of convergence α of theopenings 41 is in the range of 10° and 20°.

The typical inlet size D42 of the opening 41 (e.g. the diameter forcircular openings 41) at the feed side 42 of the perforated screen 40may be at least 2 times the typical grain size of the solidified carbondioxide.

The typical outlet size D43 of the opening 41 (e.g. the diameter forcircular openings 41) at the collection side 43 may be approximatelyequal to the mean grain size of the solidified carbon dioxide. However,according to a further embodiment, the typical outlet size D43 of theopening 41 at the collection side 43 is substantially smaller than themean grain size of the solidified carbon dioxide. The diameter D43 of acircular opening 41 at the outlet side can range from 0.5 to 5 mm thoughis preferably between 1 and 3 mm.

The depth D41 of the openings 41 measured in the direction oflongitudinal axis 44 may typically be two times the inlet size D42 ofthe opening 41. However, the depth D41 of the openings 41 may also bemore than two times the inlet size D42 of the opening 41. Preferably thedepth D41 is less than 5 times the inlet size D42.

The tapered shape and dimensions of the openings 41 allow a densepacking of solidified carbon dioxide particles to form in and possiblyabove the openings 41. In use, the solidified carbon dioxide particleswill be present in the openings 41 and on top of the perforated screen40. The dense packing of solidified carbon dioxide particles have arelatively low porosity and ensure that no leak paths are present forgases or liquids to seep through from the feed side 42 towards thecollection side 43.

Furthermore blocking said leak paths in order to obtain an impermeablelayer of solidified carbon dioxide at the perforated screen 40 may beestablished by providing means to apply static head to the solidifiedcarbon dioxide grains. The term head is used to refer to a column orlayer of liquid and solids which result in pressure on the dsolids onthe perforated screen 40.

This increases the mutual contact pressure between the carbon dioxidegrains and between the carbon dioxide grains and the side walls of theopenings 41. By increasing the cohesion and adhesion forces, the layerof carbon dioxide is made more dense.

In order to allow the solidified carbon dioxide particles to travelthrough the openings 41 towards the collection side 43 the solidifiedcarbon dioxide particles are melted from the collection side 43. Thismay be accomplished by maintaining a suitable temperature T43 at thecollection side 43 and/or maintaining a suitable pressure P43 at thecollection side 43.

The collection pressure P43 at the collection side 43 is controlled at apressure which is typically 2 bar lower than a pressure P42 at the feedside 42 and in the separation vessel 21. So, in case the separationvessel 21 is operated at a pressure of 20 bar, the pressure P42 at thefeed side is approximately equal to 20 bar and the pressure P43 at thecollection side may be controlled to be approximately 10-18 bar.

The temperature T43 at the collection side 43 of the perforated screen40 may be chosen such that given the relevant pressure, the carbondioxide is in a liquid phase. For instance for a pressure of typically10-18 bar, a temperature may be chosen between approximately −55° C. and0° C.

The temperature at the collection side may be controlled by atemperature arrangement (not shown) or by an arrangement that heats theperforated screen to a desired temperature within the liquid phase ofcarbon dioxide to melt off liquid carbon dioxide from the perforatedscreen 40.

As a result of the temperature and pressure T43, P43 at the collectionside 43 the underside of the layer of carbon dioxide that is formed willmelt and carbon dioxide will drip and may be collected in a suitablevessel or the like.

The above described embodiment provides an efficient way of separatingcarbon dioxide. By having carbon dioxide present in the solid statewithin the separation vessel 21 the carbon dioxide is separated from forinstance methane (that would otherwise mix with carbon dioxide in liquidphase). At the same time, at the collection side 43 of the perforatedscreen 40 the carbon dioxide is available in liquid phase, allowing easyfurther transportation and processing.

By providing the perforated screen 40 a solid carbon dioxide barrier isprovided between the feed side 42 and the collection side 43 allowingcontrolling the collection side and the separation side at differentconditions (pressure/temperature).

FIG. 7 shows a further embodiment.

The vessel 21 may be equipped with one or more inlets 25 which arepositioned tangent to the perimeter of the vertical section 22, suchthat a rotational flow in section 22 results. Furthermore the top gasoutlet 29 may extent as a vertical pipe in said vertical section 22 asto form a so-called vortex finder. The edge of said vortex finder is ata vertical lower position compared to the vertical position of theinlet(s) 25.

The sections 22 and 23 of vessel 21 may be physically separated by aconical shaped deflector plate or vortex breaker 30 of which the outerperimeter has a clearance C with respect to the inner perimeter of thevertical section 22. This clearance C can range typically from 0.05 to0.3 times the inner diameter of section 22.

The vortex breaker 30 breaks the rotational motion of the flow from thefirst section 22 to the collection tank 23, to prevent eddies to beformed in the collection tank 23.

Also, the vortex breaker may prevent gaseous components to travel fromthe vertical section 22 into the collection tank 23 and deflects thesegaseous components towards the top gas outlet 29.

The perforated screen 40 is now provided as part of the collection tank23. In use, a layer of CO2 will form on top of the perforated screen 40.An overflow wall 34 is formed to provide an overflow connection. Theoverflow connection allows liquids that will typically form on top ofthe layer of CO2 to pass the overflow wall 34 and leave the collectiontank 23 via fourth outlet 26.

FIG. 8 a schematically depicts a further embodiment. FIG. 8 a depicts avessel 21 and two cyclonic fluid separators 1 as described above.However, it will be understood that instead of two, any suitable numberof cyclonic fluid separators 1 may be provided.

According to this embodiment the fluid separation assembly furthercomprises a feedback conduit 81 that is on one side connected to thefourth outlet 26 and on the other side connected to a feedback inlet ofthe cyclonic fluid separator 1. The feedback conduit 81 furthercomprises a pump PU.

The carbon dioxide enriched hydrocarbon liquid components that flow viathe fourth outlet 26 are pumped by means of the pump PU through thefeedback conduit 81 to the feedback inlet of the one or more cyclonicfluid separators 1. According to FIG. 8 a, the feedback inlet isupstream of the pear-shaped central body 11 and coincides with the‘normal’ inlet 82 of the cyclonic fluid separators 1. However, thefeedback inlet may also be provided at another position, for instancehalfway the cyclonic fluid separator 1.

By providing such a feedback conduit 81, it is possible to achievepartial or even complete solidification of the CO2, without the need ofadditional cooling in the vessel 21 where the temperature reaches itslowest value. Instead the carbon dioxide enriched hydrocarbon liquidstream is first pumped to the feed pressure and combined with the streamof conduit 82 to form a new feed stream transport indicated as theconduit 81+82, where after said combined feed stream may be cooled to anew temperature which is lower than the temperature in conduit 82 andhigher than the temperature level present in the vessel 21. Typicallythe difference between the feed stream temperature in conduit 81+82 andthe temperature in vessel 21, is 25 degrees C. In order to achieve thecooling, a cooling unit 85 may be provided in conduit 81+82, as shown inFIG. 8 b.

The first outlets 6 of the cyclonic fluid separators 1 may be combinedtogether with the fifth outlet 29 of the tubular section 22 to form anoutlet 83. The fluid through the inlet 81 of the cyclonic fluidseparator 1 may comprise approximately 70% CO₂ and 30% C_(x)H_(y), whilethe outlet 83 may comprise approximately 15% CO₂ and 85% C_(x) H_(y).

Further Remarks

According to an embodiment there is provided a method of removing carbondioxide from a fluid stream by a fluid separation assembly comprising:

-   -   a cyclonic fluid separator comprising a throat portion arranged        between a converging fluid inlet section and a diverging fluid        outlet section and a swirl creating device configured to create        a swirling motion of the carbon dioxide containing fluid within        at least part of the cyclonic fluid separator, the converging        fluid inlet section comprising a first inlet for fluid        components and the diverging fluid outlet section comprising a        first outlet for carbon dioxide depleted fluid and a second        outlet for carbon dioxide enriched fluid;    -   a separation vessel having a first section in connection with a        collecting tank, said first section being provided with a second        inlet connected to said second outlet of said cyclonic fluid        separator, and said collecting tank being provided with a third        outlet for solidified carbon dioxide;        the method comprising:    -   providing a fluid stream at said first inlet, said fluid stream        comprising carbon dioxide;    -   imparting a swirling motion to the fluid stream so as to induce        outward movement of at least one of condensed components and        solidified components within the fluid stream downstream the        swirl creating device and to form an outward fluid stream;    -   expanding the swirling fluid stream, so as to form components of        liquefied carbon dioxide in a meta-stable state within said        fluid stream, and induce outward movement of said components of        liquefied carbon dioxide in said meta-stable state under the        influence of said swirling motion;    -   extracting the outward fluid stream comprising said components        of liquefied carbon dioxide in said meta-stable state from said        cyclonic fluid separator through said second outlet;    -   providing said extracted outward fluid stream as a mixture to        said separation vessel through said second inlet;    -   guiding said mixture through said first section of said        separation vessel towards said collecting tank, while providing        processing conditions in said first section such that solidified        carbon dioxide is formed out of said components of liquefied        carbon dioxide in said meta-stable state;    -   extracting the solidified carbon dioxide through said third        outlet, wherein the method further comprises:        -   forming a layer of solidified carbon dioxide extracted from            the third outlet 28 on a feed side 42 of a perforated screen            40 comprising openings 41 towards a collection side 43,        -   applying temperature and pressure conditions on the            collection side 43 of the perforated screen 40 to melt of            carbon dioxide from the layer and collect the melted carbon            dioxide through the openings 41 at the collection side 43.

The collection side 43 may be operated at a temperature and pressurecombination for which carbon dioxide is liquid. The feed side 42 may beoperated at a first pressure and the collection side 43 may be operatedat a second pressure, the second pressure being equal or lower than thefirst pressure. The temperature at the collection side 43 may be in therange of minus 55° C.-0° C., and higher than at feed side 42. Theopenings 41 have an inlet size D42 at the feed side 42 that is greaterthan an outlet size D43 at the collection side 43. The outlet size D43may be approximately equal to or substantially smaller than the grainsize of solidified carbon dioxide.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced in anotherway than described. The description above is intended to beillustrative, not limiting. Thus, it will be apparent to a personskilled in the art that modifications may be made to embodiments of theinvention as described without departing from the scope of the claimsset out below.

1. Method of removing carbon dioxide from a fluid stream by a fluidseparation assembly comprising: a cyclonic fluid separator comprising athroat portion arranged between a converging fluid inlet section and adiverging fluid outlet section and a swirl creating device configured tocreate a swirling motion of the carbon dioxide containing fluid withinat least part of the cyclonic fluid separator, the converging fluidinlet section comprising a first inlet for fluid components and thediverging fluid outlet section comprising a first outlet for carbondioxide depleted fluid and a second outlet for carbon dioxide enrichedfluid; a separation vessel having a first section in connection with acollecting tank, said first section being provided with a second inletconnected to said second outlet of said cyclonic fluid separator, andsaid collecting tank being provided with a third outlet for solidifiedcarbon dioxide, wherein said separation vessel is operated at a pressureand temperature combination that is at or in the vicinity of the phaseboundary between a vapour/liquid/solid coexistence region (IVb) and thevapour/solid coexistence region (IVa); the method comprising: providinga fluid stream at said first inlet, said fluid stream comprising carbondioxide; imparting a swirling motion to the fluid stream so as to induceoutward movement of at least one of condensed components and solidifiedcomponents within the fluid stream downstream the swirl creating deviceand to form an outward fluid stream; expanding the swirling fluidstream, so as to form components of liquefied carbon dioxide in ameta-stable state within said fluid stream, and induce outward movementof said components of liquefied carbon dioxide in said meta-stable stateunder the influence of said swirling motion; extracting the outwardfluid stream comprising said components of liquefied carbon dioxide insaid meta-stable state from said cyclonic fluid separator through saidsecond outlet; providing said extracted outward fluid stream as amixture to said separation vessel through said second inlet; guidingsaid mixture through said first section of said separation vesseltowards said collecting tank, while providing processing conditions insaid first section such that solidified carbon dioxide is formed out ofsaid components of liquefied carbon dioxide in said meta-stable state;extracting the solidified carbon dioxide through said third outlet (28).2. Method according to claim 1, wherein the method further comprises:forming a layer of solidified carbon dioxide extracted from the thirdoutlet (28) on a feed side (42) of a perforated screen (40) comprisingopenings (41) towards a collection side (43), applying temperature andpressure conditions on the collection side (43) of the perforated screen(40) to melt of carbon dioxide from the layer and collect the meltedcarbon dioxide through the openings (41) at the collection side (43). 3.Method according to any one of the claims 1-2, wherein the collectionside (43) is operated at a temperature and pressure combination forwhich carbon dioxide is liquid.
 4. Method according to any one of theclaims 2-3, wherein the feed side (42) is operated at a first pressureand the collection side (43) is operated at a second pressure, thesecond pressure being equal or lower than the first pressure.
 5. Methodaccording to any one of the claims 2-4 wherein the temperature at thecollection side (43) is in the range of minus 55° C.-0° C., and higherthan a temperature at the feed side (42)
 6. Method according to any oneof the claims 2-5, wherein the openings (41) have an inlet size (D42) atthe feed side (42) that is greater than an outlet size (D43) at thecollection side (43).
 7. Method according to claim 6, wherein the outletsize (D43) is approximately equal to or substantially smaller than thegrain size of solidified carbon dioxide.
 8. Method according to any oneof the preceding claims, wherein said extracted outward fluid stream isprovided to said separation vessel tangential to a perimeter of thefirst section, such that a rotational flow in the first section (22) isgenerated.
 9. Method according to any one of the preceding claims,wherein said first section of the separation vessel is further providedwith a fifth outlet, and said method further comprises extracting carbondioxide depleted gaseous components through said fifth outlet. 10.Method according to any one of the claims 8-9, wherein the fifth outletis formed by a vortex finder, comprising a substantially vertical pipeextending into the first section in a through an upper part of the firstsection in a downward direction, wherein the lower end of the pipe is ata vertical lower position than the second inlet.
 11. Method according toany one of the preceding claims, wherein there is provided a vortexbreaker (30) in between the first section and the collection tank. 12.Method according to any one of the preceding claims, wherein saidcollecting tank is further provided with a fourth outlet (26), and saidmethod further comprises extracting hydrocarbon liquid componentsthrough said fourth outlet (26).
 13. Method according to claim 12,wherein the hydrocarbon liquid components through the fourth outlet (26)are fed back to the cyclonic fluid separator.
 14. Method according toany one of the preceding claims, wherein said separation vessel furthercomprises a cooling arrangement configured to provide a predeterminedtemperature condition therein, said temperature condition enablingsolidification of the carbon dioxide enriched fluid.
 15. Methodaccording to any one of the preceding claims, wherein said fluidseparation assembly further comprises a scroll type discharger inconnection with said third outlet (28), and said extracting thesolidified carbon dioxide is performed by conveying by means of saidscroll type discharger.
 16. Method according to any one of the precedingclaims, wherein said fluid stream comprises a mole percentage carbondioxide larger than 10%.
 17. Method according to any one of thepreceding claims, wherein said expanding of the swirling fluid stream issuch that the swirling fluid stream reaches supersonic velocity. 18.Method according to claim 17, wherein said expanding is further suchthat a temperature below 200 K is reached.
 19. Method according to claim17 or 18, wherein said expanding is further such that a pressure isreached below 50% of a pressure at the first inlet of the cyclonic fluidseparator.
 20. Method according to any one of the preceding claims,wherein said providing said outward fluid stream as a mixture to saidseparation vessel through said second inlet is arranged such that atangential fluid stream is provided.
 21. Fluid separation assembly forremoving carbon dioxide from a fluid stream, the fluid separationassembly comprising: a cyclonic fluid separator comprising a throatportion arranged between a converging fluid inlet section and adiverging fluid outlet section and a swirl creating device configured tocreate a swirling motion of the carbon dioxide containing fluid withinat least part of the separator, the converging fluid inlet sectioncomprising a first inlet for fluid components and the diverging fluidoutlet section comprising a first outlet for carbon dioxide depletedfluid and a second outlet for carbon dioxide enriched fluid; aseparation vessel having a first section in connection with a collectingtank, said section being provided with a second inlet connected to saidsecond outlet of said cyclonic fluid separator, and said collecting tankbeing provided with a third outlet (28) for solidified carbon dioxide,wherein said separation vessel is operated at a pressure and temperaturecombination that is at or in the vicinity of the phase boundary betweena vapour/liquid/solid coexistence region (IVb) and the vapour/solidcoexistence region (IVa); wherein said fluid separation assembly isarranged to: receive a fluid stream comprising carbon dioxide at saidfirst inlet; impart a swirling motion to the fluid stream so as toinduce outward movement of at least one of condensed components andsolidified components within the fluid stream downstream the swirlcreating device and to form an outward fluid stream; expand the swirlingfluid stream, so as to form components of liquefied carbon dioxide in ameta-stable state within said fluid stream, and induce outward movementof said components of liquefied carbon dioxide in said meta-stable stateunder the influence of said swirling motion; extract the outward fluidstream comprising said components of liquefied carbon dioxide in saidmeta-stable state from said cyclonic fluid separator through said secondoutlet; provide said extracted outward fluid stream as a mixture to saidseparation vessel through said second inlet; guide said mixture throughsaid first section of said separation vessel towards said collectingtank, while providing processing conditions in said first section suchthat solidified carbon dioxide is formed out of said components ofliquefied carbon dioxide in said meta-stable state; enable extraction ofthe solidified carbon dioxide through said third outlet (28).
 22. Fluidseparator assembly according to claim 21, wherein the fluid separatorassembly further comprising: a perforated screen (40) comprising a feedside (42) and a collection side (43), the feed side (42) positioned tocollect solidified carbon dioxide from the third outlet (28), theperforated screen further comprising openings (41) towards thecollection side (42), wherein said fluid separation assembly is furtherarranged to: form a layer of solidified carbon dioxide extracted fromthe third outlet (28) on the feed side (42) of the perforated screen(40), applying temperature and pressure conditions on the collectionside (43) of the perforated screen (40) to melt of carbon dioxide fromthe layer and collect the melted carbon dioxide through the openings(41) at the collection side (43).
 23. Fluid separator assembly accordingto claim 22, wherein the collection side (43) is arranged to be operatedat a temperature and pressure combination for which carbon dioxide isliquid.
 24. Fluid separator assembly according to claims 22-23 whereinthe feed side (42) is arranged to be operated at a first pressure andthe collection side (43) is operated at a second pressure, the secondpressure being equal or lower than the first pressure.
 25. Fluidseparator assembly according to any one of the claims 22-24, wherein thefluid separator assembly is arranged to be at a temperature at thecollection side (43) that is in the range of minus 55° C.-0° C., buthigher than a temperature at the feed side (42).
 26. Fluid separatorassembly according to any one of the claims 22-25, wherein the openings(41) have an inlet size (D42) at the feed side (42) that is greater thanan outlet size (D43) at the collection side (43).
 27. Fluid separatorassembly according to claim 26, wherein the outlet size (D43) isapproximately equal to or substantially smaller than the grain size ofsolidified carbon dioxide.
 28. Fluid separator assembly according toanyone of the claims 21-27, wherein said second inlet is a tangentialinlet to a perimeter of the first section, such that a rotational flowin the first section (22) is generated.
 29. Fluid separation assemblyaccording to any one of the claims 21-28, wherein said first section isfurther provided with a fifth outlet, said fifth outlet being configuredto enable extraction of carbon dioxide depleted gaseous components. 30.Fluid separation assembly according to any one of the claims 28-29,wherein the fifth outlet is formed by a vortex finder, comprising asubstantially vertical pipe extending into the first section through anupper part of the first section in a downward direction, wherein thelower end of the pipe is at a vertical lower position than the secondinlet.
 31. Fluid separation assembly according to any one of the claims21-30, wherein there is provided a vortex breaker (30) in between thefirst section and the collection tank.
 32. Fluid separation assemblyaccording to any one of the claims 21-31, wherein said collecting tankis further provided with a fourth outlet (26), said fourth outlet (26)being configured to enable extraction of hydrocarbon liquid components.33. Fluid separation assembly according to claim 32, wherein the fluidseparation assembly comprises a feedback conduit (81), the feedbackconduit (81) being arranged to feedback the hydrocarbon liquidcomponents from the fourth outlet (26) to the cyclonic fluid separator.34. Fluid separation assembly according to any one of claims 21-33,wherein said separation vessel further comprises a cooling arrangementconfigured to provide a predetermined temperature condition therein,said temperature condition enabling solidification of a carbon dioxideenriched fluid.
 35. Fluid separation assembly according to any one ofclaims 21-34, wherein said fluid separation assembly further comprises ascroll type discharger in connection with said third outlet (28), saidscroll type discharger being configured to enable extraction of saidsolidified carbon dioxide through said third outlet (28) by conveying.36. Fluid separation assembly according to any one of claims 21-35,wherein said second inlet of said separation vessel is arranged tangentto the circumference of the separation vessel.